The invention generally relates to an energy recovery device of the positive displacement type that can be used to transfer energy from a first fluid at a higher pressure to a second fluid at a lower pressure. The invention specifically relates to the use of such an energy recover device in the process of desalination by reverse osmosis, where the device is used to transfer a portion of the energy from rejected brine to the incoming feed. Other applications include the use of the device as a fluid driven pump or a hydraulic compressor.
This invention relates to energy recovery devices, and particularly to those used in the desalination of seawater by the reverse osmosis method. The recovery problem is of vital importance in desalination by reverse osmosis. Fluid pressure energy recovered from high pressure rejected brine is utilized for the pressurization of the feed flow. Prior art energy recovery devices used in reverse osmosis systems may be classified as mechanical assistants, hydraulic driven boosting pumps and work exchangers.
A mechanical assistant commonly has the prime pump, motor and energy recovery turbine mounted on a common shaft. The turbine can either be a Pelton type or a reverse running centrifugal pump (Francis turbine). The overall efficiency of such devices is of the order of 60%.
A hydraulically driven boosting pump, sometimes called a turbocharger, is usually mounted on the same line as the primary pump in order to carry a portion of the required load. The rotating member in these devices comprises a turbine impeller fixedly coupled to a pump impeller within a common housing. This scheme has an estimated overall efficiency between 60-70%.
A work exchanger uses the rejected brine to positively pressurize an approximately equal amount of brackish feed water. One subset of this type employs a number of stationary cylinders with floating pistons and a control mechanism for synchronizing the opening and closing of valves. A second subset uses a spinning rotor with a multiplicity of channels. Work exchangers have an estimated overall efficiency between 80%-90%.
The mechanical assistants and hydraulic booster pumps involve the conversion of hydraulic energy into mechanical energy, which is then converted back to hydraulic energy. Work exchangers, on the other hand, directly transfer the hydraulic energy of one fluid (rejected brine) to hydraulic energy of the second fluid (feed), and are hence more efficient. The present invention falls within this category, i.e. a positive displacement, or work exchanger, energy recovery device. Examples of prior art devices of this sort include one taught in U.S. Pat. No. 3,791,768 which uses opposed piston/diaphragm pumps. The primary drawback of these devices is a restriction in the amount of fluid that can be handled, which renders such devices best suited to relatively small installations. Other energy recovery devices employing pistons of different areas with connecting rods are shown in U.S. Pat. No. 3,558,242 and in U.S. Pat. No. 6,017,200. Still another device of this sort uses a system of cylinders with freely moving pistons synchronized by a complex system of valves, and is shown in U.S. Pat. No. 5,797,429.
The main drawback of prior art work exchange devices is that they require a complex mechanism to control the opening and closing of valves as well as a mechanism for synchronizing various piston movements.
In addition to the energy recovery devices discussed above, there is also a class of devices in which pressure exchange takes place through direct contact between two fluid flows. Arrangements of this sort are shown in U.S. Pat. Nos. 5,988,993, 5,338,158 and 4,887,942 to Hauge. These devices have a cylindrical rotor comprising a plurality of open-ended axial channels spinning in a housing that is connected at both ends to intake and discharge ports of the differently pressurized fluids.
The main drawbacks of the prior art direct contact systems include uncontrollable internal mixing between the two flows, uncontrollable rotor speed, a complex water lubrication arrangement, axial alignment problems, lack of flexibility to deal with varying loads, and constraints on overall dimensions.
A preferred embodiment of the present invention accomplishes energy recovery through a positive displacement rotary device. In a preferred embodiment of this device, a small portion of the high pressure energy fluid is diverted through a nozzle to impinge on blades externally attached to the cylindrical rotor block, causing it to rotate. The bulk of the high pressure fluid is conveyed to axial channels within the block in order to pressurize the low pressure fluid within those channels. In some embodiments the two fluids are physically separated by freely sliding piston elements; in others no sliding elements are used and the pressure exchange is made through direct contact of the two fluids. The preferred axial channels are closed at both ends and have radially inward directed openings, one adjacent each end. Each of these openings alternately registers with axially aligned intake and discharge ports within a central stationary member so that at any given instant a single channel communicates with an intake port of one fluid and a discharge port of a second fluid. The sliding elements are arranged to freely reciprocate in respective channels in response to the alternate registering of the inward openings at the ends of the axial channels with intake and discharge ports in a central stationary member providing fluid connections for exchanging fluid flows. Each sliding element performs two strokes in the course of one complete revolution of the cylindrical block. Each stroke of the double acting sliding element comprises an intake of one fluid and a discharge of the second fluid. Alternatively, where no sliding elements are used, a fluid interface separating the two fluids acts as a sliding element.
A principal object of the present invention is to provide a device for use in a reverse osmosis desalinization plant to recover energy from waste brine flows and to deliver that energy to the feed flow.
One object of the present invention is to provide a hydraulically driven energy recovery device that does not require a separate driving means such as a motor.
Another object of the present invention is to provide an energy recovery device that does not require either a valve system or the associated electro-mechanical control mechanism needed to synchronize the opening and closing of valves.
Another object of the present invention is to provide an energy recovery device that can be used over a wide range of installation capacities.
Another object of the present invention is to provide an energy recovery device that minimizes the mixing of the two fluid flows.
Another object of the present invention is to provide an energy recovery device in which the speed of a rotating member is controlled manually by adjusting the flow rate of a fluid in a nozzle connected to an external valve.
Another object of the present invention is to provide an energy recovery device that is less costly to manufacture, easy to maintain and install in existing reverse osmosis systems than are prior art devices.
Still another object of the present invention is to provide an energy recovery device characterized by low fluid flow pulsation and vibration.
These and other objects and advantages of the present invention will be apparent from the following detailed description and the appended claims. It will be recognized that the foregoing description is not intended to list all of the features and advantages of the invention. Various embodiments of the inventions will satisfy various combinations of the objects of the invention and some embodiments of the invention will provide fewer than all of the listed features and satisfy fewer than all the listed objectives.